The present invention relates to a scanning optical system which is employed in, for example, a laser beam printer.
In a scanning optical system for a laser beam printer, a laser beam emitted by a laser diode is deflected by a polygonal mirror having a plurality of reflective surfaces to scan within a predetermined angular range. The scanning beam passes through an imaging optical system which converges the deflected laser beam to form a scanning beam spot onto a surface to be scanned such as a photoconductive surface. As the polygonal mirror rotates, the beam spot moves on the photoconductive surface. By ON/OFF modulating the beam spot as it moves, an electrostatic latent image is formed on the photoconductive surface.
One of important items to be considered in designing the scanning optical system is to prevent occurrence of ghost images on the photoconductive surface. For example, such a ghost image is caused by undesired reflections on surfaces of lenses of the imaging optical system.
More specifically, when a beam impinges on a surface of a lens provided in the imaging optical system, a potion of the beam (i.e., an undesired beam) is reflected by the surface of the lens and proceeds back to the polygonal mirror along a direction which is defined by a shape of the surface of the lens and an incident angle of the beam with respect to the surface of the lens. If the undesired beam is incident on the polygonal mirror, the undesired beam is reflected again by the polygonal mirror.
In a case where the undesired beam proceeds back to the polygonal mirror, if the undesired beam impinges on one of the reflective surfaces reflecting (i.e., deflecting) the laser beam from the light source (hereafter the one of the surface is referred to as a reflective surface A), the undesired beam reflected by the surface A may impinge on the photoconductive surface. However, in this case, the undesired beam scans on the photoconductive surface at a speed substantially the same as a scanning speed of a normal beam spot. In addition, intensity of the undesired beam is sufficiently low. Accordingly, the undesired beam reflected by the reflective surface A does not cause a problem of ghost images.
On the contrary, if the undesired beam impinges on another reflective surface (i.e., a reflective surface which is not reflecting the laser beam from the light source), the undesired beam may scan on the photoconductive surface at a relatively low speed. In this case, depending on a shape of the lens surface in the main scanning direction, the undesired beam may remain at substantially the same position on the photoconductive surface. If such a phenomenon occurs, ghost images appears on the photoconductive surface, and therefore imaging quality deteriorates.
If reflectivity of each of surfaces of lenses in the imaging optical system is zero, ghost images can be removed completely. However, in order to lower the reflectivity of the surface of the lens, a greater number of layers of coating are required. This increases manufacturing cost of the lens. It should be appreciated that to lower the reflectivity of the surface of the lens to completely zero is impossible. Therefore, it is impossible to remove the ghost images completely by using an antireflective coating.
In addition, in many cases, plastic lenses having aspherical surfaces are employed in the imaging optical system so as to reduce manufacturing cost of the scanning optical system. However, the plastic lens has a disadvantage in adhesiveness between the layers of coating and a surface of the plastic lens in comparison with glass lenses.